Signal generator



Dec. 28, 1943. A w, BARBER 2,33A5'533 SIGNAL GENERATOR 'Filed June 9, 1 941 Patented Dec. Y28, 1943 UNITED STATES ATENT @FICE SIGNAL GENERATOR Appliation June 9, 194.1,seria1'N0- 397,249

5 Claims.

vide an improved frequency modulated signal generator.

Another object is to provide improved modulation characteristics in a variable carrier frequency modulated signal generator.

Still another object is to provide means for changing the frequency range of the signal generator without aifecting its modulation characteristics.

A further object is to provide either amplitude modulation or frequency modulation or both simultaneously in a signal generator.

These and other objects will be evident from the detailed description of the invention given in connection with the drawing.

In order to design and test frequency modulation radio receivers and systems a variable frequency modulated signal generator is very useful. A freouency modulated signal is one in which the frequency is varied above and below the mean frequency at the modulation rate and in which the instantaneous frequency deviation from the mean is a linear function of the instantaneous value of the modulating voltage. A number of methods have been employed in the past for frequency modulating a signal. One method has been to vary the apparent capacity across an oscillatory circuit bv means of a thermionic vacuum tube. If a thermionic vacuum repeater tube is operated with a resistive plate load and a plate to grid. capacity the grid to cathode impedance of the tube is a capacity which varies with the gain of the tube. The gain may be varied by control of the grid voltage by means of a modulating voltage. The amount and qualitx,7 of the modulation which may be1 obtained by this method depends on the mutual conductance of the tube, the plate to grid capacity and the plate load resistor among other factors. In order to obtain large amounts of frequency deviation and which remains constant as the mean frequency is varied has presented a considerable problem in the past. The problem is further complicated if amplitude modulation is also required.

According to the present invention the frequency modulation is accomplished at a fixed frequency and hence under fixed conditions. In order to provide a variable mean frequency the modulated signal beats with a variable frequency unmodulated signal and the sum and difference frequencies are utilized. The frequencies are chosen so that the difference frequencies are chosen to provide a low frequency range as, for instance, 1 to 10 megacycles and the sum frequencies are chosen to provide a high frequency range as, for instance, 41 to 50 megacycles. Selection of the desired range is accomplished after detection so that no switching is required in the oscillator circuits. Amplitude modulation is accomplished in the detector which permits operation with frequency or amplitude modulation separately or both together.

In the drawing is shown a circuit diagram of one form of the invention.

The circuit diagram shows fixed modulated oscillator 6, variable frequency oscillator 9, modulator 5, audio frequency driver d, detector l, amplier 8 and voltmeter tube 2. Various ground points 3 are common to the system. Fixed oscillator tube 6 may be any suitable thermionic vacuum tube as for instance the beam tetrode shown having cathode 89 heated bv conventional means not shown, control grid 88, screen grid S9, and plate 9|. Grid 88 is connected to oscillator tank coil 8d through grid leak 81 by-passed by grid condenser 86. Oscillation is `produced by means of feed-back coil 83 connected to plate 9| through parasitic oscillation suppressor resistor 82 and plate bias source IM. Cathode 39 receives a proper bias by means of the cathode current flowing in resistor 92. Cathode bias resistor 92 is connected between cathode 89 and ground v3 and is by-passed by condenser 93. Screen 9|] receives a bias from source I4| and is by-passed by condenser 94. The frequency of oscillation of tube Fi is partially determined by trimmer condensers 85 and 851. The frequency of oscillation of tube i is finally determined by an effective grid to cathode capacity of modulator tube 5.

Modulator tube 5 may be any suitable tube as for instance the tetrode shown including cathode 13, control grid 12, screen grid 'H and plate 14. The effective grid to cathode capacity of tube 5 is placed acrosstank coil 8d by means of coupling condenser 8l. Cathode 13 receives its bias by means of cathode current flowing in cathode resistor 18 connected between cathode 13 and ground 3 and by-passed by condenser '19. Screen 'll receives its bias from source IM and is bypassed by means of condenser 10. The effective grid to cathode capacity of tube 5 is partly controlled by the parallel condensers 'l5 and 'I6 connected between grid l' 2 and plate 'M and by means of plate load consisting of resistor 68 and phasing coil 51 connected between plate 'I4 and bias source I4I through decoupling resistor 59. The effective grid to cathode capacity of tube 5 is also partly determined by the mutual conductance of tube 5 as determined by the net bias between control grid 'I2 and cathode i3. This effective capacity is varied by means of a modulating voltage applied to grid 'I2 through choke coil 'I'I and across radio frequency filter condenser 80.

Frequency modulation of fixed oscillator S is accomplished by means of driver tube 4 varying the grid bias of modulator tube 5. Driver tube 4 may be any suitable tube as, for instance, the tetrode having cathode 42, control grid 4|, screen grid 43 and plate 54. Driver tube 4 is essentially an amplifier and receives its input voltage from tap 40 on potentiometer 5S and feeds output transformer primary 45. Cathode 42 receives its bias from cathode current flowing in resistor 50 which is by-passed by condenser 49. The feed-back coil 47 is connected in series with cathode 42 and resistor 55 in order to decrease distortion by means of degenerative feed-back. The voltage across secondary i5 is applied to grid 12 in order to frequency modulate oscillator 5 through contacts 55 and 55 of switch 5I and radio frequency iilter 58, 55, 55 when switch is turned to FM position as shown. The deviation or amount of frequency modulation is indicated on voltmeter I5 connected across secondary 45 through switch I9, 2li, 2|. Switch I9, 25, 2| may switch a resistor in or out of the circuit to multiply the scale of meter I5. Resistor 48 is connected across secondary 45 in order to improve the regulation of the driving system and this may be necessary on large deviations since on large swings of voltage the power required to drive grid 'I2 may vary over the voltage cycle.

Audio frequency imput voltage for driver 4 may be derived from any suitable internal or external source as for instance the Audio frequency source No. l through coupling condenser 38 and switch 22, 23, 24. With switch arm 22 closed to contact 23 the input to driver 4 is essentially constant with audio frequency and may be controlled by potentiometer 3S. Since the frequency deviation of oscillator S depends on the driving audio voltage as long as the other circuit elements as, for instance, condensers 85, 851, 15, I5 and resistor 58 are held constant meter I0 may be calibrated directly in kilocycles frequency deviation. Condensers 85, B51, 15, 'I5 and resistor 58 may be varied or changed initially in order to set the deviation sensitivity, and mean frequency of the system to match the calibration of meter I 5J and to provide the desired output frequency.

Frequency modulated transmitters for broadcast service are modulated with a rising audio frequency characteristic equivalent to the curve of a circuit having a time constant of 100 microseconds. This rising characteristic may be provided by any suitable means as for instance the substantially equivalent circuit consisting of resistors 32, 34, 35, 35 and 35', condenser 33 and coil 31 all connected to selector switch 25. This circuit equivalent to a simple 1GO microsecond RL circuit is constructed by connecting resistor 32 in parallel with condenser 33 and in parallel with this, resistor 34 in series with resistor 35 in parallel with resistor 35 in series with inductance 31 and resistor 35 grounded at the connection between resistor 35 and inductance 51. Modulation following this rising characteristic may be provided by turning switch 25 to in thereby switching the above circuit in series with a suitable audio source as, for instance, Audio frequency source No. 2 and switch 22 which should be placed on contact 24 and switch I6 which should be placed on contact It. When switch 25 is in the out position this circuit for producing a rising characteristic is removed and audio source No. 2 produces a linear deviation of oscillator 6.

Up to this point the fixed oscillator and frequency modulation system has been described in detail. In order to produce a variable output frequency this modulated fixed frequency signal is caused to beat with a variable frequency signal. The variable heating frequency is supplied by tube 9 which includes cathode |53 heated by conventional means not shown, control grid |52, and plate ISI. Grid I52 is connected to tank coil |45 through grid leak |135 by-passed by grid condenser Ifl. Oscillation is produced by means of feed-back coil |53 connected to plate |5| and through filter' resistor I@ to bias source |4|. Filter resistor U52 is oy-passed by condenser |44. The 'frequency of oscillation is varied by means of turning condenser |55 and the range is set by trimmer condenser EN.

The beat between oscillators 5 and 9 is detected by detector 'I and is amplified by amplifier 8. Detector 'I includes cathode |55 which receives a bias due to cathode current in resistor 91 bypassed by condenser 58, control grid 99, screen grid itl, suppressor grid |52 and plate |03. Control grid 99 is returned to ground through resistor 55. Signal voltage from fixed oscillator 6 is placed on grid 95 by means of adjustable coupling condenser |551 and signal voltage from variable oscillator S is placed on grid 99 by means of adjustable coupling condenser I5i. The attenuation of these signals at grid 99 may be varied by means of condensers I55, |551 and 95. Plate |03 is loaded by means of resistor |94 and receives its bias from source I4 I. Detector I rectiiies the signals from oscillators 5 and 9 producing the and difference frequencies across resistor The sum and difference frequencies thus produced are amplified by means of amplifier 8. Amplifier 8 may be any suitable thermionic vacuum tube as, for instance, the pentode shown having cathode I I5 receiving its bias from the cathode current iiowing in resistor I I3 by-passed by condenser H4, control grid II5, screen grid II`I, suppressor grid I i5, and plate I I9. A portion of the sum and diiference signals appears across resistor E54 through coupling condenser |05. The amplified sum and difference signals appear across plate load resistor |25 connected between plate IIB and bias source I 4|.

The amplified sum and difference signals across resistor |25 are coupled to an output attenuator |73 and output voltmeter 2 by means of coupling condenser |2I. The output attenuator |13 includes an initial dropping resistor |55, attenuator resistors |58, ISB, |65 and ISI, output terminating resistor Il!! and push-button switches |55-|5'I--t5-I5-I5|-|52, Hill- |65 and ld-59. v

The low end of resistor |58 is shunted to ground by resistor |53 and the low end of resistor |55 is shunted to ground by resistor |57. Switch I55-I5'i-i55-I55--I5I-l2 connects the low end of resistor |55 to the high end of resistor |58 when in its neutral position as shown and connects the low end of resistor |55 to output terminal HI when in its operative position. Similarly switch IE5-|55 connects resistor |58 to resistor |55 or to output terminal I'II and switch |58|69 connectsresistor |65'to terminal in its operative position; The attenuated output appears across terminals and |12 and the full output is -available at terminal |54 connected to the high end of resistor |55. The voltage across attenuator |13 is measured by diode voltmeter 2. Diode 2 includes cathodes |35 and |36 and corresponding anodes |33 and |34. Anodes |33|34 are connected together and to the high side of resistor |55. The diode circuit includes the cathode load comprising resistor |29 and condenser |39 in shunt and paralleled by resistor |3| in series with condenser |32 and the combination is connected between paralleled cathodes K55-|36 and ground 3. Meter by-passed to ground by condenser |40 is connected in series with resistors |39 and a portion of resistor |31 to plates |33|3i and to a point on resistor VM. 'I'hus the rectified current generated by diode 2' flows thru meter il. The sensitivity of the voltmeter is adjusted by means of adjustable contact |38 on resistor |31 connected in series with meter and the zero signal current from diode 2 is bucked out by adjusting contact |75 on resistor il'l connected between battery MI and ground 3. Meter reads the voltage across the attenuator including the initial dropping resistor |55. The actual output voltage at point |54 is thus read by meter and the output voltage across points and |72 is the reading on meter times the attenuation factor introduced in attenuator |13.

Amplitude modulation of the output signal is produced by applying a modulating signal to suppressor grid |92 of detector tube '1. This is accomplished through contacts 52 and 53 of switch by throwing it to AM. 'I'his connects the output of driver tube 4 to grid |32 through filter 6|-62-63 and 64--65-65. The amount of amplitude modulation thus produced is indicated on meter I0.

With switch 5| at FD/f, switch 22 closed to 23 and switch l5 closed to il frequency and amplitude modulation are produced at the same time. Frequency modulation is produced by Audio frequency source No. 2. In this way independent frequency and amplitude modulation of the output signal is produced. The modulating frequencies may Abe anything from the same frequency to Widely different frequencies. Evidently a single source of audio frequency may also be used in which case the two modulations are equal in frequency and phase.

Thus far a generator has been described for generating a variable frequency either modulated in frequency or amplitude or both. Both sum and difference frequencies are produced in the output of detector 1. In order to provide either sum or difference frequencies in the output of the generator at will selection is made in the amplifier as, for instance, by means of selective circuits Hit-|61 and |22|23 switched in or out by means of switches MEQ and |28. In one form of the invention fixed oscillator 6 is operlated at 20 megacycles and the variable oscillator 9 is operated over a range from 21 to 30 megacycles. The plate loads of tubes l and 8 and other circuit constants are chosen so that with switch 55 on contact H0 and switch |23 on contact |21 the detector and amplifier circuits pass the difference frequency of 1 to 10 megacycles. This provides operation for one frequency range. The 1 to megacycle range is thus selected and higher frequencies are eliminated by the cut-off characteristics of the amplifier. With switch I 09 on contact |68 circuit |06 is tuned by condenser |31 to one end of the sum frequency range of 4:1 to megacycles and with switch |28 on contact |26 circuit |22 is tuned by condenser |23 near the other end of the 41 to 50 megacycle range. This provides substantially constant amplification over the sum frequency range from 41 to 50 megacycles and since circuits IBB-|01 and |22- |23 have low impedances in the 1 to 10 megacycle range the difference frequencies are eliminated. This provides a high frequency range output. It Will be noted that for convenience of calibration of the generator 1 megacycle corresponds to 4l megacycles, 2 to 42 etc. The range of operation is changed merely by shifting switches |69 and |28 in the amplifier without any connection or change in the oscillator circuits. This provides constant oscillator and modulation conditions on both ranges of output frequency.

Thus frequency modulation is accomplished at a fixed frequency at a point mid-way between the output frequency ranges. Conditions for frequency modulation are not changed by changing the range or value of the output frequency. The harmonics of the fixed oscillator fall outside both output frequency ranges. Amplitude modulation is carried out in the detector Without effect on the frequency modulation. Both frequency and amplitude modulation may be produced at the same time at the same or different modulating frequencies and by independent amounts. The output frequency ranges may be changed Without affecting either modulation or frequency stability since no changes are made in the oscillating circuits. 'I'hese and many other advantages of the system shown and described will be apparent to those skilled in the art.

While only a single embodiment of the present inventio-n has been shown and described many modifications will be apparent to those skilled in the art and within the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

l. In a signal generator, the combination of, an oscillator having a fixed mean frequency, means including a second oscillator for frequency modulating said oscillator about said mean frequency, a variable frequency oscillator, a detector for rectifying signals from the first mentioned oscillator and from said variable frequency oscillator to produce sum and difference frequencies, means including said second oscillator for amplitude modulating said sum and difference frequencies after they have been produced, means for selecting said sum frequencies and eliminating said difference frequencies to produce a first useful output signal, means for selecting said difference frequencies and eliminating said sum frequencies to produce a second useful output, and means for selecting said selecting means.

2. In a signal generator, the combination of, a high frequency electronic oscillation generator, a low frequency electronic oscillation generator, means for frequency modulating said high frequency oscillation in accordance with a signal derived from said low frequency generator, a variable frequency second high frequency electronic oscillation generator, means for mixing signals from said two high frequency generators to produce a frequency modulated variable frequency beat note signal, and means for amplitude modulating said beat note signal in accordance with a second signal derived from said low frequency generator to provide a frequency and amplitude modulated test signal.

3. In a signal generator, the combination of.

an oscillator having a fixed mean frequency, means including a second oscillator for frequency modulating said oscillator about said mean frequency, a variable frequency oscillator, a detector for rectifying signals fromI the first mentioned oscillator and said variable frequency oscillator to produce sum and difference frequencies, means including said second oscillator for amplitude modulating said sum and difference frequencies after they have been produced, means for selecting said sum frequencies and eliminating said difference frequencies to produce a useful output signal, the fixed mean frequency of the first means oscillator being greater than the range of the variable frequency oscillator.

4. In a signal generator, the combination of, an oscillator having a fixed mean frequency, means including a second oscillator for frequency modulating the first mentioned oscillator about said mean frequency, a variable frequency oscillator having a range less than the mean frequency of the first mentioned oscillator, a detector for rectifying signals from the rst mentioned oscillator and the variable frequency oscillator to produce sum and difference frequencies, means including said second oscillator for amplitude modulating said sum and difference frequencies after they have been produced, and means for selecting said difference frequencies and eliminating said sum frequency to produce a useful output signal.

5. In a signal generator, the combination of, an oscillator having a fixed mean frequency, means including a second oscillator for frequency modulating the first mentioned oscillator about said mean frequency, a variable frequency oscillator having a range less than the mean fre quency of the first mentioned oscillator, a detector for rectifying signals from the first mentioned oscillator and the variable frequency oscillator to produce sum and difference frequencies, means including said second oscillator for amplitude modulating said sum and difference frequencies after they have been produced, and means for selecting a said sum frequency and eliminating a said difference frequency to produce a useful output signal.

ALFRED W. BARBER. 

